Some types of implantable medical devices, such as cardiac pacemakers or implantable cardioverter defibrillators, provide therapeutic electrical stimulation to a heart of a patient via electrodes of one or more implantable leads. The therapeutic electrical stimulation may be delivered to the heart in the form of pulses or shocks for pacing, cardioversion, or defibrillation. In some cases, an implantable medical device may sense intrinsic depolarizations of the heart, and control the delivery of therapeutic stimulation to the heart based on the sensing. Some implantable medical devices provide cardiac sensing functionality without delivery of therapy. Some implantable medical devices are used to provide therapy and/or monitoring for any of a variety of conditions, including neurological or gastrological systems, as examples.
Cardiac resynchronization therapy (CRT) is one type of therapy delivered by an implantable medical device. Cardiac resynchronization therapy may help enhance cardiac output by resynchronizing the electromechanical activity of the ventricles of the heart. Ventricular desynchrony may occur in patients that suffer from congestive heart failure (CHF).
Implantable medical devices are typically powered by internal batteries, and battery depletion is inevitable. Many implantable medical devices are provided with the ability to communicate a “recommended replacement time” (RRT). The RRT informs the clinician that the device's power supply is nearing, but has not yet reached end-of-service (EOS), the point at which the power supply cannot provide sufficient energy to keep the device operable. The advance warning provided by an RRT gives the clinician the opportunity to take appropriate measures (e.g., to replace the device prior to EOS). Some implantable medical devices derive estimates of remaining battery life, which may include periodic measurements of battery voltage and either, or both of, battery impedance and current drain.